HW 01 - Hello R!

The main goal of this assignment is to introduce you to R and RStudio, which we will be using throughout the course both to learn the statistical concepts discussed in the course and to analyze real data and come to informed conclusions.

Note

R is the name of the programming language itself and RStudio is a convenient interface.

An additional goal is to introduce you to the process for generating a report that you will upload on Canvas to turn in this and other assignments.

As the homeworks progress, you are encouraged to explore beyond what the homework dictate; a willingness to experiment will make you a much better programmer.

Before we get to that stage, however, you need to build some basic fluency in R.

For this assignment we begin with the fundamental building blocks of R and RStudio: the interface, reading in data, and basic commands.

Getting started

Going forward each assignment will start with a “Getting started” section but for this one we will just jump in.

Warm up

Before we introduce the data, let’s warm up with some simple exercises.

YAML

Note

The top portion of your R Markdown file (between the three dashed lines) is called YAML. It stands for “YAML Ain’t Markup Language”. It is a human friendly data serialization standard for all programming languages. All you need to know is that this area is called the YAML (we will refer to it as such) and that it contains meta information about your document.

Open the R Markdown (Rmd) file in your project, change the author name to your name, and knit the document.

Packages

In this homework we will work with two packages: datasauRus which contains the dataset we’ll be using and tidyverse which is a collection of packages for doing data analysis in a “tidy” way.

These packages are already installed for you.

You can load the packages by running the following in the Console.

library(tidyverse)
library(datasauRus)

Note that the packages are also loaded with the same commands in your starter R Markdown document.

Data

The data frame we will be working with today is called datasaurus_dozen and it’s in the datasauRus package.

Actually, this single data frame contains 13 datasets, designed to show us why data visualization is important and how summary statistics alone can be misleading.

The different datasets are marked by the dataset variable.

To find out more about the dataset, type the following in your Console: ?datasaurus_dozen.

A question mark before the name of an object will always bring up its help file.

This command must be ran in the Console.

Note

If it’s confusing that the data frame is called datasaurus_dozen when it contains 13 datasets, you’re not alone! Have you heard of a baker’s dozen?

Exercises

Exercise 1

  • Based on the help file, how many rows and how many columns does the datasaurus_dozen file have?
  • What are the variables included in the data frame?
Important

Add your responses to the starter file for this homework.

Next, let’s take a look at what these datasets are.

To do so we can make a frequency table of the dataset variable:

datasaurus_dozen %>%
  count(dataset)
# A tibble: 13 × 2
   dataset        n
   <chr>      <int>
 1 away         142
 2 bullseye     142
 3 circle       142
 4 dino         142
 5 dots         142
 6 h_lines      142
 7 high_lines   142
 8 slant_down   142
 9 slant_up     142
10 star         142
11 v_lines      142
12 wide_lines   142
13 x_shape      142
Note

The original Datasaurus (dino) was created by Alberto Cairo in this great blog post. The other Dozen were generated using something called “simulated annealing.”1

1 Source: Matejka, Justin, and George Fitzmaurice. “Same stats, different graphs: Generating datasets with varied appearance and identical statistics through simulated annealing.” Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems. ACM, 2017.

Exercise 2

  • Plot y vs. x for the dino dataset.
  • Then, calculate the correlation coefficient between x and y for this dataset.

Below is the code you will need to complete this exercise.

Basically, the answer is already given, but you need to include relevant bits in your starter Rmd document and successfully knit it and view the results.

Start with the datasaurus_dozen and pipe it into the filter function to filter for observations where dataset == "dino".

Store the resulting filtered data frame as a new data frame called dino_data.

dino_data <- datasaurus_dozen %>%
  filter(dataset == "dino")

There is a lot going on here, so let’s slow down and unpack it a bit.

First, the pipe operator: %>%, takes what comes before it and sends it as the first argument to what comes after it.

So here, we’re saying filter the datasaurus_dozen data frame for observations where dataset == "dino".

Second, the assignment operator: <-, assigns the name dino_data to the filtered data frame.

Next, we need to visualize these data.

We will use the ggplot function for this. Here is the code. In what follows, I will again break it down for you:

ggplot(data = dino_data, mapping = aes(x = x, y = y)) +
  geom_point()

The first argument of ggplot() is the data you’re visualizing (dino_data).

Next we define the aesthetic mappings. In other words, the columns of the data that get mapped to certain aesthetic features of the plot, e.g. the x axis will represent the variable called x and the y axis will represent the variable called y.

Then, we add another layer to this plot where we define which geometric shapes we want to use to represent each observation in the data. In this case we want these to be points, hence geom_point.

Note

If this seems like a lot, it is. You will learn about the philosophy of building data visualizations in layer in detail next week. For now, follow along with the code that is provided.

For the second part of this exercise, we need to calculate a summary statistic: the correlation coefficient.

Correlation coefficient, often referred to as \(r\) in statistics, measures the linear association between two variables.

You will see that some of the pairs of variables we plot do not have a linear relationship between them.

This is exactly why we want to visualize first: visualize to assess the form of the relationship, and calculate \(r\) only if relevant.

In this case, calculating a correlation coefficient really doesn’t make sense since the relationship between x and y is definitely not linear – it’s dinosaurial!

But, for illustrative purposes, let’s calculate the correlation coefficient between x and y.

Tip

Start with dino_data and calculate a summary statistic that we will call r as the correlation between x and y.

dino_data %>%
  summarize(r = cor(x, y))
# A tibble: 1 × 1
        r
    <dbl>
1 -0.0645

🧶 ✅ At this stage, you might want to Knit and view the changes if you have not already. Make sure that everything is looking legit in the rendered output.

Exercise 3

  • Plot y vs. x for the star dataset. You can (and should) reuse code we introduced above, just replace the dataset name with the desired dataset.
  • Then, calculate the correlation coefficient between x and y for this dataset.
  • How does this value compare to the r of dino?

Exercise 4

  • Plot y vs. x for the circle dataset. You can (and should) reuse code we introduced above, just replace the dataset name with the desired dataset.
  • Then, calculate the correlation coefficient between x and y for this dataset.
  • How does this value compare to the r of dino?

Exercise 5

  • Finally, let’s plot all datasets at once. In order to do this we will make use of faceting.
Tip

Facet by the dataset variable, placing the plots in a 3 column grid, and don’t add a legend. See code below for examples of how to do it.

ggplot(datasaurus_dozen, aes(x = x, y = y, color = dataset))+
  geom_point()+
  facet_wrap(~ dataset, ncol = 3) +
  theme(legend.position = "none")

And we can use the group_by function to generate all the summary correlation coefficients.

datasaurus_dozen %>%
  group_by(dataset) %>%
  summarize(r = cor(x, y))

One last thing!

You’re done with the data analysis exercises, but we’d like you to do two more things:

  • Resize your figures:

Click on the gear icon (next to the Knit icon) in on top of the R Markdown document, and select “Output Options…” in the dropdown menu.

In the pop up dialogue box go to the Figures tab and change the height and width of the figures, and hit OK when done.

Then, knit your document and see how you like the new sizes. Change and knit again and again until you’re happy with the figure sizes. Note that these values get saved in the YAML.

You can also use different figure sizes for different figures.

To do so click on the gear icon within the chunk where you want to make a change.

Changing the figure sizes added new options to these chunks: fig.width and fig.height.

You can change them by defining different values directly in your R Markdown document as well.

  • Change the look of your report:

Once again click on the gear icon in on top of the R Markdown document, and select “Output Options…” in the dropdown menu.

In the General tab of the pop up dialogue box try out different Syntax highlighting and theme options.

Hit OK and knit your document to see how it looks.

Play around with these until you’re happy with the look.

Wrapping up

Yay, you’re done! But to submit this assignment to Canvas, you need to Knit the document to a PDF file output. Luckily, this is very easy to do.

One option is to select the “Knit” icon’s dropdown menu and then select the option for knitting to PDF. When you do so, a PDF document should be automatically generated in the same folder where you found the HW starter file.

Another option is to change the YAML from output: html_document to output: pdf_document and click the “Knit” button as you have been doing.

Either way, once you’ve generated the PDF, submit it to Canvas!